Advance Journal of Food Science and Technology 5(9): 1249-1254, 2013
ISSN: 2042-4868; e-ISSN: 2042-4876
© Maxwell Scientific Organization, 2013
Submitted: June 17, 2013
Accepted: June 28, 2013
Published: September 05, 2013
Study on Buttresses Distance of Gas Pipelines in the Deviated Well Based on Stress
Analysis Method
1
1
Kun Huang, 1Hongfang Lu, 1Kunrong Shen, 2Haowen Shu and 1Yijia Wang
School of Petroleum Engineering, Southwest Petroleum University, Chengdu Sichuan 610500, China
2
Southwest Pipeline Branch of Petro China Company Limited, Chengdu Sichuan 610041, China
Abstract: As the stress is one of the main factors affecting the safe operation of the pipeline in the gas pipeline
tunnel crossing project, in order to ensure the safe operation of the pipeline, it is necessary to research the stress
conditions of the gas pipelines in the deviated well. In this study, we discuss on these two aspects of the gas
pipelines in the deviated well: the necessity of adding buttresses and the distance between buttresses. We do the
numerical simulation of China Shaanxi-Beijing III gas pipeline in the Eighth Fort Yellow River tunnel areas using
stress analysis software CAESAR II; exploring its secure and economical buttresses distance. Our research shows
that the pipeline can be safely run after adding buttresses and it is more reasonable to set buttresses distance of 20 m
for the numerical example. Our research provides a reference for the design of the buttresses distance of gas
pipelines in the deviated well and has some engineering value.
Keywords: Buttresses distance, CAESAR II, deviated well, gas pipeline, stress analysis, tunnel
INTRODUCTION
With the continuous development of the longdistance pipeline construction, most of the natural gas
pipeline through the complex geological environment,
the inevitable through obstacles, such as crossing rivers,
swamps, climb mountains. Tunnel crossing overcome
elevation and terrain obstacles, not only reduces the
destruction of vegetation and soil erosion, reduce the
difficulty of pipeline construction, but also save the
pipe material, reducing construction costs. Increasingly
sophisticated pipe tunnel crossing technology is
becoming one of the special sections of pipeline
construction and reliable method, which deviated well,
is one of the tunnel crossing forms.
Pipeline failure has many reasons, besides the
construction and material defects, corrosion, third-party
damage, design defects, misuse, geological disasters,
pipeline fatigue failure and other reasons, there is an
important reason is that the stress is higher than the
design strength of pipeline requirements arising from
the failure of damage.
The general crossing methods are shield method
and mining method when crossing the mountains and
large rivers, the structural form of tunnel is “deviated
well-drift-deviated well”. According to the current
situation and research data, tunnel uses the form of pipe
rack support and import and export will be, respectively
installed with fixed buttresses, which are used to block
the outside pipelines’ effect on the tunnel.
Design of the tunnel crossing pipe installation is
divided into three stages: choice of pipeline installation,
piping arrangement and pipeline stress analysis. Terrain
limitations and maintenance inconvenience lead to
more serious the consequences of the accident for the
crossing pipeline. Crossing pipeline failure for many
reasons, in addition to the unreasonable design,
construction quality problems, failure of pipeline
corrosion, pipeline fatigue failure and other reasons,
there is another important reason is that the bends
cannot meet the strength requirements. Pipeline design
is the basis of the quality of construction, the
reasonableness of pipeline design is directly related to
the structural safety of the pipeline (CNPC, 1995).
Currently, scholars did very little research on
pipeline stress analysis, so it is necessary to research the
stress conditions of the gas pipeline. However, the
design of the tunnel crossing structure only rely on
empirical parameters, so the pipe stress analysis should
be taken seriously in the pipeline design.
Currently, more widely pipe stress analysis
software CAESAR II, AutoPIPE and Triflex which
CAESAR II’s application is the most widely used.
COADE developed for the preparation of CAESAR II
has a powerful static and dynamic calculation and
analysis capabilities. It includes pipeline combined load
stress calculation and analysis, container nozzle
flexibility and stress check analysis, natural frequency,
time history analysis, its theory is one-dimensional
beam element finite element method. Its pipe stress
check method followed by the relevant provisions of
the American National Standards B 31., (The American
Society of Mechanical Engineers, 2010).
Corresponding Author: Hongfang Lu, School of Petroleum Engineering, Southwest Petroleum University, Chengdu Sichuan
610500, China
1249
Adv. J. Food Sci. Technol., 5(9): 1249-1254, 2013
In this study, we discuss on these two aspects of
the gas pipelines in the deviated well: the necessity of
adding buttresses and the distance between buttresses.
By discussing these two aspects, we can clear the stress
distribution of the pipeline in the deviated well and
determine safe, economical construction program.
In order to explore the necessity to adding
buttresses and the most reasonable distance between
buttresses, we analyze the stress conditions of ShaanxiBeijing III gas pipeline in the eighth Fort Yellow River
tunnel areas using stress analysis software CAESAR II.
Comparing the stress conditions of pipe without any
constraints with the pipe with buttresses, we can draw
the necessity of adding buttresses. Discussion on the
buttresses distance, we analyze the different buttresses
distance of straight pipeline and came to security,
economic buttresses distance.
MATERIALS AND METHODS
Pipe stress analysis and calculations: The basic stress
of the pipe is axial stress, shear stress, hoop stress and
radial stress. Axial stress is the normal stress which is
parallel to the tube axis. Shear stress is a variety of load
which may make the crystal adjacent planar sliding
trend. Hoop stress caused by pressure, parallel to the
tangent of the pipe wall circumference. The radial stress
caused by the internal pressure, the direction parallel to
the radius of the tube. Figure 1 shows the pipeline
stress:
Axial stress:
a 
Fax Pd 0 M b


Am
4t
Z
(1)
 max 
M rC
R
(3)
Hoop stress:

 h  P  ri 2 

ri 2 r02 

r2 
r
 ri 2 
ri 2 r02 

r2 
r
 ri 2 
2
0
(4)
Radial stress:

 r  P  ri 2 

where,
Fax =
Am =
P
=
do =
t
=
Mb =
Z
V
Q
Mr
=
=
=
=
C
=
R
ri
r0
r
=
=
=
=
2
0
(5)
The cross-section of the internal force, N
The cross-sectional area of the wall, m3
Internal pressure, Mpa
Outer diameter, m
Wall thickness, m
Bending moment acting on the cross-section,
N·m
Section modulus in bending, m3
Shearing force, N
Shear coefficient
Torsional moment acting on the cross-section,
N·m
Distance of the point of the cross section to the
torsion center, m
Torsional sectional coefficient
The inner radius of the pipe, m
The outer radius of the pipe, m
The radial position of the stress calculation
point, m
Shear stress:

Caused by a shearing force acting in cross-section:
 max 

VQ
Am
Caused by the torsional load:
(2)
Check criterion of the gas pipelines: According to the
basic characteristics of the stress, which fall into three
categories, including primary stress, secondary stress
and operating stress. Among them, primary stress is
normal stress or shear stress in the pipe caused by
external load and it does not have characteristic of self
limiting and mainly brings about plastic failure.
Fig. 1: Pipe stress diagram
1250 Adv. J. Food Sci. Technol., 5(9): 1249-1254, 2013
Secondary stress is normal stress or shear stress caused
by constraints due to the deformation of the pipe and it
has characteristic of self-limiting and localization and
mainly leads to fatigue failure. Operating stress is the
maximum stress value of local stress concentration due
to the sudden change of load and abrupt deformation of
shape and structure. And it can lead to brittle fracture
and fatigue failure.
Therefore, during the design process, it is not
feasible to rely only on whether the stress under
operating conditions reaches limit value or not to the
pipeline safety, but to preset an allowable stress [σ] as
the criteria to verify the strength requirements within
the pipe fittings limit stress range, in which:
[σ] = F߶ߪ௦
where,
F = Strength design coefficient of different region
area
߶ = Weld coefficient
Due to thermal expansion and contract, curvature
mutation, endpoint additional displacements or
restraints, pipeline will be loaded with the
corresponding axial stress, shear stress, bending
moment and moment of torque. Generally, stress
calculation needs the checking of primary stress,
secondary stress and the operating stress.
Primary stress of pipe should not exceed allowable
stress at the pipe design temperature, σL≤ [σ].
Secondary stress should not exceed the allowable stress
range σE≤σallow = f (1.25σc + 0.25 [σ]). If σh>σL, the
allowable stress range is σallow = f [1.25 (σc + σh) - σL].
Operating stress σOPE is the sum of σL and σE, under the
condition that σL + σE≤σs.
where,
σh = The allowable stress of the pipe design
temperature, Mpa
σL = The sum of longitudinal bending stress, which
is the sum of loads caused by pressure such as
the outside carrier of longitudinal stress, gravity
and wind, Mpa
σE = Secondary stress, Mpa
σallow = The allowable stress range
σc = The allowable stress in 20C pipe, Mpa
σs = The minimum yield strength of pipe materials
standard, Mpa
σOPE = Operating stress, Mpa
f
= Reducing coefficient, considering the allowable
stress range influenced by the total cycle time in
life expectancy
CAESAR II software checking method is based on
the American ASME B31.8 Gas Transportation and
Distribution Piping Systems, it is recommended that
pipeline stress should not exceed the 90% of the
pipeline allowable stress (Tang, 2003).
The method of modeling: A complete pipeline model
includes three parts: Piping input, add constraints and
define the load conditions.
Piping input: Piping input is the basis of stress
analysis, which includes pipeline parameters (DX, DY,
diameter, etc.), pipeline’s route in the space (CosX,
CosY, etc.), external conditions (temperature, pressure,
fluid density, etc.), pipeline material parameters
(allowable stress, Poisson's Ratio) and constraints
(ANC, Y, etc.).
Add constraints: The actual project, the pipeline
system is composed of a variety of devices and
supporting accessories, so modeling need to add a
variety of constraints to limit the displacement of the
pipe. In software, we need to input the constraint node
and the type of constraints.
The definition of the conditions:



Operating conditions: (OPE) L1 = W + T + P, the
operating condition is the maximum regional
concentration stress value caused by load,
structure shape.
Primary stress calculation condition: (SUS)
L2 = W + P, caused by inner pressure and weight.
Secondary stress calculation condition: (EXP)
L3 = L1 - L2, caused by the thermal stress.
where,
W = Weight load
T = Temperature load
P = Pressure load
Case study of deviated well gas pipelines: China
Eighth Fort Yellow River tunnel is located in the
junction of Shaanxi and Shanxi. According to the
design information, total length of the tunnel is 1598 m,
uses the form of pipe rack support (Fig. 2b) and each
end of the pipe will be, respectively installed with fixed
buttress 1 and 4 (Fig. 2a), which are used to block the
outside pipelines’ effect on the model. The pipe length
along the tunnel entrance is 35 and 20 m-long pipeline
deviates from the west side of the tunnel portal about
60. The length of the west inclined tunnel is 465 m,
longitudinal gradient is 28. The length of the east
inclined tunnel is 409 m, longitudinal gradient is 25.
And the length of the drift is 644 m. The pipe length
along the tunnel export is 45 and 20 m-long pipeline
1251 Adv. J. Food Sci. Technol., 5(9): 1249-1254, 2013
(a) Fixed buttress simplify model
(b) Buttress simplified model
(c) Anchor block
Fig. 2: Model simplification diagram
Fig. 3: Pipeline model diagram
Table 1: Pipeline parameters
Material
Diameter (mm)
API×70
1016
Fluid density (kg/m3)
Insulating layer thickness (mm)
95
0
Table 2: Bends parameters
Location
Bend A, bend B
Bend C, bend D
Wall thickness of straight pipe (mm)
26.2
Pressure (MPa)
10
Specification
߶1016×33.1, angle is 28
߶1016×33.1, angle is 25
Wall thickness of pipe bend (mm)
33.1
Temperature (C)
40
Quantity
2
2
Corrosion (mm)
1
Allowable stress (MPa)
485
Remark
X70 steel, R = 6D
X70 steel, R = 6D
deviates from the east side of the tunnel portal about
60°. Midpoint of the west inclined tunnel and east
inclined tunnel will be respectively installed with
anchor block 2 and anchor block 3 (Fig. 2c). Curvature
radius of the hot-bending bends is R = 6 D, D is the
pipe diameter.
Figure 2 shows the pipeline constraints of
simplification model. Figure 3 shows the pipe model.
Table 1 show the design data of gas pipelines in
deviated well. Table 2 shows the parameters of bends.
RESULTS AND DISCUSSION
Analysis of the necessity to add the buttresses:
Design two models, one is the deviated well pipeline
doesn’t add any support, another is the pipeline add
buttresses, the distance between two buttresses is 20 m.
Fig. 4: The operating stress ratio of the two models
1252 Adv. J. Food Sci. Technol., 5(9): 1249-1254, 2013
Table 3: Buttress distance and quantity
Buttresses distance (m)
10
Quantity
57
12
48
14
41
16
36
18
32
Table 4: The average stress ratio
Buttresses distance (m)
Average stress ratio (%)
12
52.46
14
53.15
16
53.91
18
54.83
10
51.69
20
29
20
55.82
Fig. 5: The operating stress ratio of the six models
We set pipe length as the abscissa, the operating stress
ratio as the ordinate, then draw two model's operating
stressratiographical sheet tomake a comparison (Fig. 4).
From Fig. 2 we arrive at that the pipe stress value
of the pipeline not added any supporting exceeds the
allowable value of the stress and the maximum stress
achieves a rate of 5915.87%. The difference is the pipe
stress value of the pipeline added the buttresses does
not exceed 0.9 [σ] = 434369.7000 MPa, so the model
meet the strength requirements, the highest operating
stress ratio of the pipeline is 97.32%, which happened
at the bend B (Huang et al., 2012; Sha et al., 2013;
Wang, 2002).
Analysis of the buttresses distance: In order to
explore the safety, economic buttress distance, we use
the straight pipe intercept method in this study. Select
the level of the pipe model straight pipe which is drift
and analyze the pipe stress value variation of the
different buttresses number. The total length is 644 m,
we design different buttresses distance model: 10, 12,
14, 16, 18 and 20 m, respectively. In order to reduce the
impact of the fixed buttresses at both ends, the shortest
distance between buttresses which are close to fixed
buttress is 40 m (Wu et al., 2012).
The same, we set pipe length as the abscissa, the
operating stress ratio as the ordinate, then draw these
six model's operating stress ratio graphical sheet to
make a comparison (Fig. 5). As shown in Table 3,
different buttress distance is corresponding to the
number of buttress (Liu et al., 2012).
We can be drawn from Fig. 5 and Table 4 that
different buttresses distance pipelines' stress value did
not exceed 0.9 [σ] = 434369.7000 MPa, so these
models meet the strength requirements. And with the
increase of the number of buttresses, the average stress
decreased. The maximum average stress ratio was
55.82% (buttresses distance of 20 m); minimum
average stress ratio was 51.69% (buttresses distance of
10 m) (Wu and Tong, 2003).
CONCLUSION
In this study, we analyze the necessity of adding
buttresses and the distance between buttresses of the
gas pipelines in the deviated well. The analysis showed
adding buttresses can greatly reduce the stress value
and ensure the safe and stable operation of the
pipelines. For buttress distance analysis, from the
experimental results, we have come to set buttress
distance of 10 m can better ensure the safe operation of
the pipeline, its average stress ratio was 51.69%. The
pipeline, with 20 m buttresses distance, has the largest
average stress ratio, which is 55.82%, but only a
difference of 4.13 with 51.69%. Having regarded to the
economic and practical, the fewer the number of
buttresses, the smaller the investment. Buttresses
distance can be set according to the actual situation, it is
recommended to set the buttresses distance to 20 m. In
this study, our research provides a reference for the
design of the buttresses distance of gas pipelines in the
deviated well and has certain versatility.
1253 Adv. J. Food Sci. Technol., 5(9): 1249-1254, 2013
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